Winterstrike. Liz Williams

Winterstrike spy Hestia Mar has been sent to Caud to recover details of an ancient weapon. During her stay in the Martian city, she encounters the ghost of a warrior, who turns out to be the encoded representation of the city's bombed library. Hestia Mar manages to access the library's data, but realizes too late what she has done: by downloading the information, she has virtually guaranteed the use of the weapon against Caud by her own government. Desperate to rescue the situation she makes her way back home across the dangers of the Crater Plain.
Meanwhile, in Winterstrike itself, the festival of Ombre has been taking place upon the eve of war. Hestia's cousin Shorn -imprisoned by her family for accidentally consorting with a male - manages to escape. Her sister Essegui, pursuing her to the dangerous mountains of Mars, discovers a plot by creatures who hold the secrets of the Martian past, and its future. While Essegui battles forces back in Winterstrike, Hestia travels to Earth in an attempt to save her city . . .
'Williams's prose is unfussy and compelling . . . this is adventurous, thought-provoking science fiction' "The Times"